Display devices using feedback enhanced light emitting diode

ABSTRACT

Display devices using feedback-enhanced light emitting diodes are disclosed. The display devices include but are not limited to active and passive matrix displays and projection displays. A light emissive element disposed between feedback elements is used as light emitting element in the display devices. The light emissive element may include organic or non-organic material. The feedback elements coupled to an emissive element allow the emissive element to emit collimated light by stimulated emission. In one aspect, feedback elements that provide this function include, but are not limited to, holographic reflectors with refractive index variations that are continuous.

CROSS-REFERENCE TO RELATED) APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 15/225,002, filed on Aug. 1, 2016, which is a divisional ofU.S. patent application Ser. No. 14/811,170, filed on Jul. 28, 2015,which is currently pending, which is a divisional of U.S. patentapplication Ser. No. 10/434,326, filed on May 8, 2003, now U.S. Pat. No.9,129,552, issued on Sep. 8, 2015, and claims priority to U.S.Provisional Patent Application Ser. No. 60/379,141, filed on May 8,2002, all of which are herein incorporated by reference in theirentirety.

TECHNICAL FIELD

The present application relates to display devices, and particularly, todisplay devices using feedback enhanced light emitting diodes.

BACKGROUND

Display devices in use today typically employ liquid crystal displays(LCDs) and more recently, organic light emitting devices (OLEDs). FIGS.1a and 1b illustrate examples of display devices such as active andpassive matrix display devices and their operations briefly.

Matrix displays typically contain a grid of small picture elements(pixels), which can be switched to form characters and display graphicsand video images. The electrodes are patterned as a series of stripes,with the stripes 11, 12 on one glass piece running perpendicular to thestripes on the other glass piece. The electrodes are made from atransparent, conductive material, usually indium-tin oxide (ITO).Switching cells or pixels 10 are formed where the stripes overlap asshown in FIG. 1a . In liquid crystal displays, the pixels are comprisedof liquid crystal material sandwiched between the electrodes. In OLEDs aseries of layers of organic semiconductor material, one of which emitslight on application of current, are sandwiched between the electrodes.

Passive matrix displays use a simple grid to supply the charge to aparticular pixel. That is, the rows or columns are connected tointegrated circuits that control when a particular column or row isbiased with the proper display drive voltage. To turn on a pixel, theintegrated circuit biases the correct column and the correct row withthe drive voltage signals. The row and column intersect at thedesignated pixel, and the row and column bias voltage result in thecorrect voltage at that pixel.

In active matrix displays a drive scheme is used that employs a storagecapacitor 14 and a transistor switch 13 at each pixel site as shown inFIG. 1b . Active matrix displays most commonly use thin film transistors(TFT). The TFTs, usually microscopic in size are arranged in a matrix ona glass substrate and connected to the row and column busses as shown inFIG. 1b . To address a particular pixel, the proper row is biasedswitching on the TFT gates connected to that row. Then the correctcolumn is biased with the proper drive voltage. Since all of the otherrows that the column intersects are turned off, only the storagecapacitor at the designated pixel receives a charge. The storagecapacitor 14 is able to store electrical charge and hold the biasvoltage on the pixel 15 after the TFT gate is switched off and until thenext refresh cycle. This means that the signal does not have to berefreshed as often and thus larger matrixed arrays are possible. Inaddition, the transistor prevents crosstalk by only switching on thepixel when the full switching voltage is applied.

The display devices including the above described matrix displaydevices, however, have problems associated with them such as poorviewing characteristics, for example, in high ambient illuminationenvironments, poor visibility over wide viewing angles, and/or highpower consumption. Accordingly, there is a need for more efficientdisplay devices.

SUMMARY

Display devices using feedback-enhanced light emitting diodes areprovided. A display device in one aspect comprises a luminescent devicecomprising at least one layer of light emissive material disposedbetween a first feedback layer and a second feedback layer. At least oneof the first feedback layer and the second feedback layer may comprisematerial having at least in part periodically varying refractive indexprofile. An imaging element is provided proximate to the second feedbacklayer to allow projection of displayed images at a distance. An imagingelement may comprise a projection lens, a light diverging screen, adiffusing screen or some other type of rear projection screen, or anyother image forming apparatus.

A display device in another aspect comprises a luminescent devicecomprising at least one layer of light emissive material disposedbetween a first feedback layer and a second feedback layer. At least oneof the first feedback layer and the second feedback layer may comprisematerial having at least in part periodically and continuously varyingrefractive index profile.

Further features as well as the structure and operation of variousembodiments are described in detail below with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b illustrate examples of prior art matrix display devices.

FIG. 2 illustrates an emissive device that may be used in the displaydevices of the present disclosure in one embodiment.

FIG. 3 illustrates a feedback-enhanced light emitting diode in oneembodiment that is used in the display devices of the presentdisclosure.

FIG. 4 illustrates an example of a color passive matrix display in oneembodiment.

FIG. 5 illustrates an example of an active matrix display usingfeedback-enhanced OLED.

FIG. 6 illustrates an example of a feedback-enhanced active matrix OLEDthat includes an allowance for dead space in one embodiment.

FIGS. 7 and 8 illustrate examples of an active matrix drive in severalembodiments of the present disclosure.

FIG. 9 illustrates a projection system including the OLED in oneembodiment.

FIG. 10 illustrates a laser projected image exiting an OLED array andimmediately entering the screen structure creating an image on thescreen front surface of the rear projection screen in an active matrixdevice.

FIG. 11 illustrates a laser-projected image exiting an OLED array andimmediately entering the screen structure creating an image on thescreen front surface of the rear projection screen in a passive matrixdevice.

DETAILED DESCRIPTION

Display devices using feedback-enhanced light emitting diodes aredisclosed. Display devices include, but are not limited, to active andpassive matrix displays, and projection systems. Feedback-enhanced lightemitting diode is fully disclosed in co-pending U.S. patent applicationSer. No. 10/434,326 entitled “FEEDBACK ENHANCED LIGHT EMITTING DEVICE,”filed on May 8, 2003. For Example, FIGS. 2 and 3 illustrate afeedback-enhanced light emitting diode in one embodiment that is used inthe display devices of the present disclosure.

FIG. 2 illustrates an emissive device in one embodiment. The device 1includes an emissive layer 2 and a feedback element 4. The feedbackelement 4 may be a layer with at least in part periodic index ofrefraction variation that allows some light to be transmitted throughthe feedback element 4. In another aspect, the feedback element 4 may bea layer with periodic and continuous index of refraction variation. Afeedback element layer with a periodic and continuous index ofrefraction variation is described in detail in co-pending U.S. patentapplication Ser. No. 10/434,326, entitled “FEEDBACK ENHANCED LIGHTEMITTING DEVICE.”

A second feedback element 6 may also be included such that the emissivelayer is between the two feedback elements 4, 6. The second feedbackelement 6 may allow some light to be transmitted through the secondfeedback element 6 or substantially reflect the light incident upon it.In one embodiment, a structure with a periodic index of refractionvariation, a plane mirror, a distributed Bragg reflector (DBR), oranother reflector may be used as the second feedback element 6.

The device in one aspect may also include other elements such atransparent buffer layer lying between the feedback layers and theemissive layer, a diffuser, an anode, a cathode or other elements. FIG.3 illustrates an emissive device 300 having additional elements. Forexample, a pair of electrodes such as a cathode 102 and an anode 104 maybe placed between an emissive layer 2 and the feedback layers 46,respectively.

The cathode 102 may include a transparent conductive structure with alow work function surface adjacent to the emissive layer 2 such that itis able to inject electrons into the emissive layer 2. In one aspect,for the cathode 102 to have the desired transparency, a two-layercathode may be provided. The two-layer cathode may include a very thin,for example, 5 nanometer (nm.) metal cathode such that the metal isessentially transparent. The metal may then be backed, for example, onthe feedback layer side, with a transparent conductor like indium-tinoxide (ITO) to yield high enough conductivity to have a low impedancedevice. The anode 104 may include a transparent conductive materialchosen to have a high work function such that it is able to inject holesinto the emissive layer 2.

The emissive layer 2 may include an electroluminescent material whosespectral emission band overlaps the reflection bands of the feedbacklayers 4 and 6. In one aspect, the emissive layer 2 may also be afluorescent or phosphorescent emissive material, an emissive inorganicsemiconductor material (such as GaAs, AlGaAs, or InGaN), anorganometallic material, a composite organic/inorganic material, or aliquid crystalline material.

The feedback layers 4 and 6 may include light non-absorbing materialwith a periodically varying index of refraction. The feedback layers 4and 6 may act as a photonic crystal that reflects light of a givenwavelength band propagating along the axis labeled “light out” 110. Aphotonic crystal is a material that because of a periodically varyingrefractive index along one or more axes cannot support light propagationof particular frequencies along those axes. In sufficient thickness itthus becomes a perfect reflector over some reflection band along thoseaxes and is said to have a photonic band gap in light energies it isincapable of supporting. Feedback layers 4 and 6 show a one-dimensionalphotonic band gap.

Another way of looking at this is a that the light entering the feedbacklayer material along the layer normal axis suffers a small reflectioneach time it passes through one cycle of the refractive indexoscillation. When the feedback element is thick enough, the feedbackelement may act as a nearly perfect reflector at the resonantwavelength, 2d, where d is the pitch of the refractive index spatialoscillation.

The feedback layers in one aspect are fabricated from planewaveholograms with peak reflectivity at the desired emission wavelength.

In one aspect, the device 300 shown in FIG. 3 may be inverted. That is,the position of the cathode 102 and the anode 104 may be interchanged.

The device 300 also may include a substrate 106 placed adjacent to afeedback layer, for example the feedback layer 6. The substrate 106 isused as a layer on which the device 300 may be built. In one aspect thesubstrate 106 may be comprised of a transparent material. In one aspect,a material may be applied over the device 300 to function as a cover108. The cover 108, for example, functions to hermetically seal outambient water and oxygen, or otherwise to protect the device 300 fromchemical or other degradation.

Other components of the device 300 may include a hole transport layerbetween the anode 104 and the emissive layer 2. The hole transport layermay be used to allow more electron/hole recombination to occur at theemissive layer 2. For example, in emissive layers having imbalancebetween electron and hole mobilities, usually with low hole mobilities,the electron/hole recombination tends to occur at the anode. Similarly,a device with a direct anode/emitter interface tends to be inefficientbecause many traps, that is, sites at which non-radiative de-excitationof the emitter occurs, exist at the emitter/anode interface. Using holetransport layers, for example, with high hole mobilities minimizes theproblem of the electron/hole recombination occurring at the anode. Thehole transport layer may also be chosen to have a hole conduction bandintermediate between those of the anode 104 and the emissive layer 2,thus providing more efficient hole injection from the anode into theemitter.

A hole injection layer may also be provided between the anode 104 andthe hole transport layer. For example, if anode materials likeindium-tin oxide (ITO) having less than well defined band structuresthat may lead to inefficient hole injection into the device are used,hole injection layers like copper phthalocyanine may be provided tobetter define band structure with energy level intermediate between ITOand hole transport materials. Providing the additional hole injectionlayers thus may assist hole injection and produce a more efficientdevice.

In another embodiment, additional hole transport layers may be insertedbetween the hole injection layer and the emitter to further smooth outband energy differences. If the hole transport layer adjacent to theemitter has its electron conduction band at an energy level nearly thesame as the emitter, electrons can “overshoot” the emitter withrecombination occurring in the transport layer rather than the emitter.This overshoot may be eliminated by interposing an electron blockinglayer that has a high energy electron conduction band, but good holeconduction, between the emitter and the transport layer.

In another embodiment, an electron transport layer may be providedbetween the cathode 102 and the emissive layer 2. The electron transportlayer performs the similar function for electrons that the holetransport layer performs for holes. As with hole transport layers,additional electron transport layers may be added to assist band energymatching.

In another embodiment, an electron injection layer may be providedbetween the cathode 102 and the electron transport layer. Ideally, it isdesirable to have as low a work function material for the cathode aspossible so that energy is not expended injecting electrons into thedevice. Very low work function metals such as calcium may be used.Calcium, however, may be very chemically reactive and very sensitive tomoisture and oxygen. Aluminum also may be used. Although aluminum hashigher work function, it has been found that overcoating the aluminumwith a very thin film of materials like lithium or magnesium fluorideprovides a “band bending” effect that helps relieve the band energymismatch.

In another embodiment, a hole blocking layer may be provided between theemitter and hole transport layer to eliminate hole “overshoot” from theemitter. The above described carrier transport, injection, and blockinglayers are also typically used in the conventional OLED devices.Accordingly, further details of these elements will not be describedherein.

In one embodiment, the device 300 may also include a buffer layer, forexample, a clear dielectric interposed between an electrode and afeedback layer. When the buffer layer is placed between the cathode 102and the feedback layer 4, it may act as a hermetic barrier between thecathode and the outside environment especially during subsequentprocessing. The buffer layer also may provide the right size spacingbetween the two feedback layers such that destructive interference oflight in the gap between the two feedback layers does not occur. Toachieve this function, the buffer layer may be inserted between thefeedback layer and the electrode to adjust the optical thickness of thedevice. The buffer layer may also be used to maintain the proper phaserelationship between the refractive index profiles in the two feedbacklayers. In addition the buffer layer may be used to adjust the thicknessof the gap between the feedback layers thereby tuning the wavelengths ofthe modes of the light that is resonating in the gap.

The devices shown in FIGS. 1 and 2 substantially reduce or eliminate thelight losses due to total internal reflections that would otherwiseoccur at the refractive index mismatch at boundaries. This approximatelydoubles the amount of light extracted from the device through thesubstantial elimination of light absorption loss inside of the device.

In one aspect, referring back to FIG. 2, the feedback elements 4, 6located on either side of the emissive layer 2 form a resonant cavity.The feedback elements 4, 6 reflect light back into the material of theemissive layer 2 and allow stimulated emission to occur when sufficientlight is reflected into the emissive layer 2. For example, the number ofinteractions between photons and excitons regulate the rate ofstimulated emission. Thus, by localizing light in the resonant cavityand thus causing a high density of photons at the emissive layer 2, avery rapid stimulated emission conversion may be produced.

Typically, without the induced stimulated emission, spontaneousemission, which is a relatively slow and purely statistical process,dominates the light generation process in an emissive material. Therapid conversion of excitation energy to light by stimulated emissionleaves the spontaneous emission process with little or no excited stateenergy to convert to light. An even slower process, non-radiativede-excitation, converts excited state energy to heat. Thus, stimulatedemission preempts conversion of excited state energy to heat since themechanism of heat formation is orders of magnitude slower than that ofstimulated emission. Consequently, the excited state energy of thedevice 1 is converted predominantly into light, not heat. The consequentreduction in heat generation also results in reduced temperature in thedevice, which allows for a longer life and more efficiency in thedevice.

In one embodiment of a feedback enhanced light emitting display the oneor more device cathodes and the one or more anodes overlay each other ina pattern such that the areas of overlap, and when properly energized,display visual information.

In matrix displays using light-emitting diodes, for example, organiclight-emitting diodes, the organic light-emitting layer is generallydivided into individual pixels. The pixels are generally arranged inorthogonal rows and columns and may be switched between emitting andnon-emitting states by altering the current flow through them. Thepixels are generally controlled via a passive or active matrixarrangement. In a passive matrix device one of the electrodes ispatterned in rows and the other in columns. Each pixel may be caused toemit light by applying an appropriate voltage between the row and columnelectrodes at whose intersection it lies. An active matrix deviceemploys at least one capacitor and at least one transistor at each pixelsite.

FIG. 4 illustrates an example of a color passive matrix display in oneembodiment. The display shown is an organic light emitting diode (OLED)display using holographic feedback layers. The holographic feedbacklayers may be produced by applying an approximately ten micron thicklayer 101 of the holographic recording material to a glass substrate102. In this example, the material is then exposed three times to arequired plane wave interference patterns through, for example, threeseparate photomasks to create a feedback hologram with an area for red(660 mn) light 103, an area for green (515 nm) light 104, and an areafor blue (440 nm) light 105. The holographic medium is then fixed sothat no further photochemical modification occurs. Next, aluminum isvacuum evaporated onto the surface of the holographic medium andpatterned into anode buses 106. In one embodiment, reference element 106may be a metallization that forms padouts at the ends of the striped ITOanodes and also provide a narrow bus that runs the length of each anodestripe along one of its edges to provide low impedance connection. Then,a layer of a transparent conductor 107, such as indium-tin oxide (ITO)is vacuum deposited on top of the holographic feedback layer. The ITO isphotolithographically patterned into striped anodes lying on top of andparallel to the striped red, green, and blue feedback areas. In oneembodiment, the ITO is made as thin as possible, for example,approximately 500 angstroms (Å) so as to minimize light absorption inthe laser stack. The metal anode buses 106 are applied to minimize thevoltage drop across the ITO.

In this example, the OLED materials are then patterned on top of theanode structure. The OLED may be composed of from one to five or morelayers, for example, 109-115 of organic semiconductor. The OLEDmaterials may be low molecular weight, polymeric, another suitablematerial or a combination of these materials. The low molecular weightmaterials may be vacuum deposited and patterned by the use of a shadowmask or by a mushroom process using patterned photoresist as an in situshadow mask. The mushroom process is described in Society forInformation Display International Symposium, May 2000, Seminar LectureNotes. Volume 1, p. M-3/40. Polymers and some low molecular weightmaterials may be deposited by solvent casting. In this case, thematerials may be patterned using ink jet printing.

In this example, photoresist is first applied and then patterned to forma crossover insulator 108. Next, a very thin, hole injection layer 109of copper phthalocyanine, for example, approximately 50 Å is uniformlydeposited through a shadow mask onto the red, green, and blue anodes.Without moving the shadow mask, a 350 Å layer of N,N′-Dinaphthalen-1-yl-N, N′-diphenylbenzidene (NPB) hole transport layer110 is deposited onto the anodes. Next, a 200 Å layer 111 ofparahexaphenyl (PHP) emissive layer is deposited through a shadow maskonto the blue anodes only. The green emissive layer 112 is formed bydepositing a 200 Å layer of tris-(8-hydroxyquinoline) aluminum (Alq3)through a shadow mask onto the green anodes only. The red emissive layer113 is formed by depositing a 200 Å layer of5,10,14,20-tetraphenylporphine (TPP) through a shadow mask onto the redanodes only.

Next, the electron transport layer 114 of 450 Å of 2-(biphenyl-5-(4-tertbutylphenyl)-1,3,4-oxadiazole) (PBD) is vacuum deposited through ashadow mask onto the red, green, and blue sub-pixels. Without moving theshadow mask used to deposit the electron transport layer an electroninjection layer 115 of 50 Å of lithium fluoride is deposited. Next,approximately 50 Å layer 123 of aluminum is deposited through the shadowmask to form the cathode. Alternatively, a metal-free cathode structuremay be formed to avoid the light absorption of even very thin layers ofcathode metals that are detrimental to the lasing action of the device.The metal free cathode layer may be composed of 50 Å of bathocuproine orsome other transparent conductive organic material. A more detaileddiscussion of a metal-free electrode can be found in G. Gu et al.,Journal of Applied Physics 86, p. 4067 (1999), the entire disclosure ofwhich is incorporated herein by reference.

On top of the aluminum, 500 Å of ITO 116 is sputter deposited through ashadow mask to form the cathode electrodes. Aluminum is evaporated ontothe entire device and then patterned into cathode bus lines 117 usingphotolithography with a boron trichloride plasma etch. To protect OLEDstructures during further processing a 1000 Å thick silicon nitridelayer 118 is vacuum deposited onto the entire substrate.

In this example, the device is removed from vacuum and the holographicrecording material used to create the ten-micron thick feedback layer onthe cathode side of the device is applied by solvent casting. In asimilar manner to the film on the opposite side of the device, thefeedback layer is successively patterned with an area 119 optimized for660 nm wavelength light (red), with an area 120 optimized for 515 nmwavelength light (green), and with an area 121 optimized for 440 nmwavelength light (blue). The display is then encapsulated by applicationof a coverglass 122 with an epoxy peripheral seal.

In one embodiment of the present disclosure, emitter material ispatterned photolithographically. In one embodiment, emitters used forpatterning photolithographically include but are not limited to emittersthat are photo-cross-linkable.

FIG. 5 illustrates an example of an active matrix display using afeedback-enhanced electroluminescent device. For example, afeedback-enhanced active matrix device may be fabricated like the deviceof FIG. 4, with the starting substrate including previously fabricatedactive matrix drive. The current switched drive of the active matrix mayuse a two or four thin film transistor (TFT) architecture or anotheractive matrix OLED drive architecture may be used. In one aspect, theactive matrix structure differs from the passive matrix structure ofFIG. 4 in that the feedback layer/electroluminescent deviceconfiguration structure includes a connection of the thin filmtransistor (TFT) sources 150 to the corresponding pixel anodes 151. Theanode feedback layer 152 is patterned so as to allow access to the TFTsource metallization. The metallization (FIG. 4, 106) that followsdeposition of the anode feedback layer and the anode ITO in passivelyaddressed displays, in one embodiment, may be used to provide the pixelanode to TFT source interconnect 153. For example, metallization 106 mayextend down the side of the mesa of feedback layer material 152. Thus106 may be extended into and/or become 153. In one embodiment, the anodeITO electrode 151 is patterned into the shape of the pixel with theaddition of an interconnect pad to one side instead being a continuouscolumn or row electrode. The patterning of the feedback layer may beaccomplished in a number of ways. For example, the feedback structuresmay be printed by ink-jet printing techniques or an alternative printingtechnique in one embodiment. In another embodiment, the layer may bedeposited as a continuous film and then patterned lithographically. Thisis possible for example, because the recording material isphoto-patternable. In another embodiment, the layer may be plasma etchedthrough a lithographically patterned mask layer.

In one embodiment, the light emitting device layers 155-165 arepatterned as striped layers covering a whole column of pixel elementsjust as they were in the passive display of FIG. 4. The cathode ITOstripes 164 and aluminum cathode buses 165 are formed as in FIG. 4,except the cathode ITO stripes 164 and the aluminum cathode buses 165are column electrodes and buses as opposed to rows. Additionally, thedisplay may be overcoated with a planarizing layer 166 and/or a siliconnitride protective layer 167 to insure that the feedback layer is ofuniform thickness. The planarizing layer 166 and the silicon nitrideprotective layer 167 are applied before applying and exposing thecathode feedback layer with red 168, green 169, and blue (not shown)optimized areas.

The interconnect 153 of the active matrix, feedback-enhanced OLEDdisplay of FIG. 5 may be fabricated on the side of an approximately tenmicron thick mesa of holographic recording material 152. If the side ofthe mesa is too much of a vertical step, there may be openings in someinterconnections. In this case, an allowance for dead space between theTFT and the anode electrode so as to allow a moderate slope on the mesaedge may be included.

FIG. 6 illustrates an example of a feedback-enhanced active matrix OLEDthat includes an allowance for dead space in one embodiment. In thisembodiment, the TFTs 182, the display row and column buses arefabricated on raised ribs 180 patterned on the display glass substrate.The anode feedback layer 181 then may be patterned into roughlyrectangular honeycomb depressions interspersed between an x-y grid ofribs 180. This configuration may be useful if the anode feedback layerholographic recording material is ink-jet printed since the ribs maythen constrain the deposited droplet of recording material solution fromspreading. If the ribs 180 are extended a little higher, the ribs 180also may be used to contain droplets of solution from ink-jet printedlight emitting device layer components 183.

The ribs 180 may be photolithographically patterned on the glasssubstrate from a high temperature resistant polymer or from sol-gelglass. It is also possible to pattern the ribs 180 from a thickmetallization layer overcoated with an insulating material, for example,to avoid shorting. In this structure, the metal rib cores may be usedfor heat dissipation in displays with intense thermal loads, forinstance, in projection displays.

In another embodiment, display resolutions may be improved by buildingthe TFT matrix on top of an already deposited and exposed holographicfeedback layer using a very low temperature polycrystalline silicon TFTprocess such as used with flexible polymer substrates. Examples of verylow temperature process can be found in C. S. McCormick, C. E. Weber, J.R. Abelson, and S. M. Gates, “An amorphous silicon thin film transistorfabricated at 125 degrees Celsius by dc reactive magnetron sputtering,”Appl. Phys. Lett., Vol. 70, no. 2, pp. 226-7 and P. M. Smith, P. G.Carey, and T. W. Sigmon, “Excimer laser crystallization and doping ofsilicon films on plastic substrates,” Appl. Phys. Lett., Vol. 70, no. 3,pp. 342-344, 1997.

In this method of producing a feedback enhanced OLED, the stimulatedemission produced feedback enhancement does not strongly depend on thethickness uniformity of the OLED display device semiconductive orconductive layers, their parallelism, or their surface finish becausethe lasing action is defined by the phase relationship of the feedbacklayers. In the case of holographic feedback layers, maintenance of thecorrect phase relationship between the two feedback layers may beensured by the phase-locking methods described in a copendingapplication entitled “FEEDBACK-ENHANCED LIGHT EMITTING DEVICE.” Thisensures that the set of interference fringes created in space by theexposure apparatus is aligned with one feedback layer while the otherfeedback layer is recorded.

The active matrix thin film transistors 170 in FIG. 5 may be formed fromconventional Poly-SI TFTs in one embodiment. In another embodiment, theactive matrix element may be formed from other kinds of three terminalswitching elements. In another embodiment, materials used for formingthe TFTs or other switching elements may have amorphous or singlecrystal structure and may be formed from materials other than silicon.

FIGS. 7 and 8 illustrate examples of an active matrix drive in severalembodiments of the present disclosure. Each pixel element in a multi-rowFE-OLED display of the present disclosure may be addressed with a drivesignal continuously. As shown in FIG. 7, the active matrix drive in oneembodiment comprises two thin film transistors (TFTs) 704, 706, astorage capacitor 702, and an FE-OLED device 712 at each pixel site. Thesource electrode of the TFT 704 is connected to a data line 710 and thedrain electrode of the TFT 704 is connected to the gate electrode of theTFT 706. The gate electrode of the TFT 706 is connected to the storagecapacitor 702. The FE-OLED device 712 is connected to the drainelectrode of the TFT 706. A scan line 708 allows pixels in individualdisplay rows to be addressed one row at a time. When the gate of TFT 704is enabled data line 710 establishes the gate bias voltage on TFT 706.This bias in turns meters current flow through TFT 2 706 from source todrain thus establishing the current level in the light emitting deviceor OLED pixel and controlling the level of light output from the pixelbased on the luminance versus current characteristic of the device. TheTFTs 704 706 provide a current to the device 712, for example, a FE-OLEDdevice and serve as an active driving device. The capacitor 702 storesthe driving signal charge.

FIG. 7 shows a simple two transistor configuration scheme for latching adesired grayscale current level into an OLED active matrix pixel. Inanother embodiment, a four transistor autozeroing pixel driveconfiguration shown in FIG. 8 may be used. This configuration may beused, for example, in cases in which TFT to TFT variation inpolycrystalline silicon active driving matrices may lead to pixel topixel variation in current level for the same gate bias on transistor,the (See R. Dawson. et al., “Design of an Improved Pixel for aPolysilicon Active-Matrix Organic LED Display”, SID InternationalSymposium Proceedings, 1998, p. 11; R. Dawson. et al., “The Impact ofthe Transient response of Organic Light Emitting Diodes on the Design ofactive Matrix OLED Displays”, IEEE International Electron DeviceMeeting, 1998, p. 875; R. Dawson, et al., “A Poly-Si Active-Matrix OLEDDisplay with Integrated Drivers’, SID International SymposiumProceedings, 1999, p. 438; R. Dawson and M. Kane, “Pursuit of ActiveMatrix Organic Light Emitting Diode Displays”, SID InternationalSymposium Proceedings, 2001, p. 372).

Here the addition autozero (AZ) 824 and autozero bar (AZB) 820 lines andtransistors TFT 3 814 and TFT 4 826 allow measurement of variations inthe threshold of TFT 2 806 before the gate of TFT 1 804 is enabled toallow the data voltage to bias the gate of TFT 2 806. Storage of thethreshold voltage in capacitor C1 816 then allows the gate bias of TFT 2806 to be offset to allow for threshold voltage variation. The abovedescribed active matrix addressing configurations using TFTs aretypically used in conventional active matrix OLED devices. (See forinstance, Fish, et al., “A Comparison of Pixel Circuits for ActiveMatrix Polymer/Organic LED Displays”. SID International SymposiumProceedings, 2002, p. 968; and S. Tam, et al., “Poly-Si Driving Circuitsfor Organic EL Displays”, Paper 4925-20, Conference 4925A, ElectronicImaging 2001). Accordingly, further details of these elements will notbe described herein.

In one embodiment, the matrix addressing scheme in the OLED displaydevices of the present disclosure allows for maintaining constantcurrent on a selected pixel element. Various constant current levels maybe maintained on pixels to support gray scale operations. Such grayscale operation may be performed by active matrices fabricated frompolycrystalline silicon or single crystal silicon materials. The grayscale modulation is accomplished by a combination of analog currentadjustment and/or time modulation of drive current at individual pixelsduring each addressing timeframe of the graphic information.

In another embodiment, display devices in the present disclosure mayprojection systems that use feedback enhanced light emitting device.Such projection systems may include but are not limited to displaymonitors and televisions. FIG. 9 illustrates an example of a projectionsystem 200 that includes an FE-OLED micro-laser array 202, for example,with a projection lens 204 to project an image on a screen 206 accordingto another embodiment of the present disclosure. FIG. 9 is illustratedwith using an FE-OLED device as an example. The device shown in FIG. 9also may use other feedback enhanced light emitting devices, and are notlimited to light emitting devices using an organic material. Aprojection system that includes a micro-laser array 202 simplifies theprojection optics of the projection since substantially collimated imagelight is generated. This may obviate the need for expensive collimationoptics with all of the associated problems and reduce number ofcomponents in the projection system 200. The reduced number ofcomponents substantially reduce the cost, complexity and size of theprojection system 200. The projection system of this embodiment also maynot need an additional color separation apparatus used in projectionsystems, for example, films, color wheels, and the like. The projectionlens 204 may be replaced by more sophisticated projection optics such asa compound lens or lens system, for example, to eliminate aberrationsand other unwanted artifacts in the projected image. Furthermore,additional optics, for example, mirrors, may be introduced between theprojection lens and the screen to fold the projection path such that theentire system may be contained in an enclosure of small size.

FIGS. 10 and 11 illustrate direct-view flat panel displays utilizingFE-OLEDs in which a laser projected image exiting an OLED array andimmediately entering the screen structure creating an image on thescreen front surface of the rear projection screen. As in FIG. 9, FIGS.10 and 11 illustrate examples using FE-OLEDs. The devices shown in FIGS.10 and 11 also may use other feedback enhanced light emitting devices,and are not limited to light emitting devices using an organic material.FIG. 10 illustrates an active matrix device while FIG. 11 illustrates apassive matrix device. The rear projection screen may be a rearprojection screen built according to U.S. Pat. Nos. 5,563,738 and5,481,385. In one embodiment, the projection screen may be placed asmall distance away from the emissive device. This may be useful inproviding mechanical design flexibility in display installation. Theprojection screen may be of the scattering type, refractive ordiffractive types or any combination thereof.

In the example of a direct-view active matrix FE-OLED display 1000 shownin FIG. 10, a TFT-based pixel drive circuit 1002, for instance,described with reference to FIG. 8 provides drive current through ananode bus 1004 to an OLED structure 1006 disposed between two feedbacklayers, the back feedback layer 1008 and the front feedback layer 1010.The two feedback layers (1008 and 1010) form part of a feedback enhancedlight emitting device, for example, an FE-OLED. Light from the OLEDstructure 1006 is emitted out of the front surface 1012 of the frontfeedback layer through front coverglass 1014. In the figure blue light1016, red light 1018, and green light 1020 are emitted from threedifferent adjacent FE-OLED structures that are configured to emit red,green, and blue light respectively. In this way a color matrix displaymay be built up. Light, for example, 1018, is incident on the rearsurface of a rear projection screen 1022.

The screen comprises an array of tapered micro-light guides 1024comprised of an optically transparent material. The interstitial areasbetween the light guides are filled with a black-filled resin 1026, forinstance, filled with carbon black, that has a lower refractive indexthan the micro-light guide material. As a result light 1018 is multiplyreflected out through the light guides and out through the light guidetips 1028. The geometry of the reflections, and for example, optionallyroughening of the light guide tips, leads to a wide angular dispersal ofthe light 1030 exiting the screen. The screen 1022 may be bonded to theFE-OLED array by an adhesive 1040 or separated from it by an air gap.

This direct-view active matrix FE-OLED display may be used under highambient illumination because ambient light 1032 striking the frontsurface of the display assembly may be highly likely to be absorbed inthe black resin 1026 that constitutes on the order of 90% of the frontsurface 1034 of the screen. The small amount of ambient light that doespass through the light guide tips 1028 for the most part enters theFE-OLED) structure through surface 1012. Feedback layers 1008 and 1010may be designed to have spectrally narrow reflection bands. Therefore,less than half the light striking surface 1012 is directly reflectedback towards screen 1022 and the display viewer. The remainder of thelight transits the FE-OLED and impacts potting material 1036 that fillsthe space between the FE-OLED and the back substrate 1038. Pottingmaterial 1036 may be filled with a black filler so that the remainingambient light that impinges it is almost completely absorbed. The lightabsorbing function of potting material 1036 may also be achieved by athin layer of light absorbing material deposited between pottingmaterial 1036 and the FE-OLED structure. A black matrix material mayalso be deposited over the top of the active matrix pixel drive circuit1002 to further enhance ambient light absorption.

A result of the highly efficient absorption of ambient light by thedisplay structure in FIG. 10 may mean that the display has what iscalled a “dead front” that reflects almost no ambient illumination. Thisin turn means that the display will be easily readable under very highlevels of ambient illumination even if the display drive current turneddown to the low levels that lead to long service life.

The direct-view passive matrix FE-OLED display 1100 shown in FIG. 11comprises components that function similarly to the components describedwith reference to FIG. 10. In FIG. 11, a matrix of anode bus lines 1102and cathode bus lines 1104 may be used in place of the TFT pixel drivecircuitry shown in FIG. 10. The OLEDs 1006, the feedback layers 1006 and1008, the substrates 1014 and 1038, and the various components in therear projection screen 1022 function similarly as described withreference to FIG. 10. The interaction of the display 1100 with incomingambient light 1032 also is similar.

The embodiments described above are illustrative examples and it shouldnot be construed that the present disclosure is limited to theseparticular embodiments. Various changes and modifications may beeffected by one skilled in the art without departing from the spirit orscope of the invention as defined in the appended claims.

We claim:
 1. A display device, comprising: a first feedback layeradapted to receive and reflect light; one or more first electrode stripsformed over the first feedback layer; one or more semiconducting layersformed over the first electrode strips, at least one of the one or moresemiconducting layers comprising at least a luminescent material; one ormore second electrode strips formed over the luminescent material thesecond electrode strips being formed such that they overlay one or moreof the first electrode strips, wherein an area where the first electrodestrip and the second electrode strip overlap comprises a segment areacapable of being driven via the first electrode strips and the secondelectrode strips; a second feedback layer adapted to receive and reflectlight disposed over the one or more second electrode strips; and animaging element disposed proximate to one of the first feedback layerand the second feedback layer; wherein both the first feedback layer andthe second feedback layer have refractive index profiles that vary atleast in part periodically along an axis normal or substantially normalto a plane of the respective feedback layer; and wherein the one or moresemiconducting layers comprising at least a luminescent material lie inan optical cavity between the first and second feedback layers.
 2. Adisplay device, comprising: a first feedback layer adapted to receiveand reflect light; one or more transparent or substantially firstelectrode strips formed over the first feedback layer; one or moresemiconducting layers formed over the first electrode strips, at leastone of the one or more semiconducting layers comprising at least aluminescent material; one or more transparent second electrode stripsformed over the luminescent material the second electrode strips beingformed such that they overlay one or more of the first electrode strips,wherein an area where the first electrode strip and the second electrodestrip overlap comprises a segment area capable of being driven via thefirst electrode strips and the second electrode strips; a secondfeedback layer adapted to receive and reflect light disposed over theone or more second electrode strips; and an imaging element disposedproximate to one of the first feedback layer and the second feedbacklayer wherein the one or more semiconducting layers comprising at leasta luminescent material lie in an optical cavity between the first andsecond feedback layers.
 3. A display device, comprising: a firstfeedback layer adapted to receive and reflect light; one or more firstelectrode strips formed over the first feedback layer, one or moresemiconducting layers formed over the first electrode strips, at leastone of the one or more semiconducting layers comprising at least aluminescent material; one or more second electrode strips formed overthe luminescent material the second electrode strips being formed suchthat they overlay one or more of the first electrode strips, wherein anarea where the first electrode strip and the second electrode stripoverlap comprises a segment area capable of being driven via the firstelectrode strips and the second electrode strips; and a second feedbacklayer adapted to receive and reflect light disposed over the one or moresecond electrode strips; and wherein both the first feedback layer andthe second feedback layer have refractive index profiles that vary atleast in part periodically along an axis normal or substantially normalto a plane of the respective feedback layer; and wherein the one or moresemiconducting layers comprising at least a luminescent material lie inan optical cavity between the first and second feedback layers.